1 

THE EFFECT OF INSTALLATION 

LOCATION ON RAILROAD HORN 

SOUND LEVELS 

Amanda S. Rapoza 

Gregg G. Fleming 

John A. Volpe National Transportation Systems Center 

55 Broadway 

Cambridge, MA 02142-1093 USA 

Phone: (617) 494-2372 

Fax: (617) 494-3208 

E-mail: rapoza@volpe.dot.gov 

ABSTRACT 

Many comments have been received as a result of the Federal Railroad Administration’s (FRA) 

issuance of a Proposed Rule for the Use of Locomotive Horns at Highway-Rail Grade Crossings. 

A large group of comments were received on a particular provision within the rule, stating that 

the sound level generated by the horn, when measured at the side of the locomotive, shall not 

exceed the sound level measured in front of the locomotive. In the late 1980’s it became the de 

facto standard to install horns on the top/center portion of the locomotive. This was done in an 

attempt to reduce the noise exposure for the locomotive cab occupants. However, the result was 

that measured sound levels off to the side of the locomotive were often higher than levels in front 

of the locomotive. Consequently, this provision in the FRA’s Proposed Rule may force railroad 

operators to relocate many installed horns. While supporting comments were made by many 

municipalities and individuals, negative comments were also received on this provision. 

In order to document precisely the effect of horn placement on the locomotive, a series of tests 

were conducted. These tests measured the sound level around the locomotive for five types of 

locomotive horns, mounted in four locations on two locomotives. By measuring and 

documenting the variation in sound level around the horn and locomotive in a consistent manner, 

the differences in sound level output as a function of distance and the differences in noise 

exposure levels can be assessed. 
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1.0 INTRODUCTION 

Many comments have been received as a result of the Federal Railroad Administration’s (FRA) 

issuance of a Proposed Rule for the “Use of Locomotive Horns at Highway-Rail Grade 

Crossings”

1

. One particular provision states that the sound level generated by the horn, when 

measured at the side of the locomotive, shall not exceed the sound level measured in front of the 

locomotive. In the late 1980’s it became the de facto standard to install horns on the top/center 

portion of the locomotive. This was done in an attempt to reduce the noise exposure for the 

locomotive cab occupants. However, the result was that measured sound levels off to the side of 

the locomotive were often higher than levels in front of the locomotive. Consequently, this 

provision in the FRA’s Proposed Rule may force railroad operators to relocate many installed 

horns. A number of negative comments were also received on this provision. 

In response to these comments and concerns the FRA, in conjunction with the Volpe National 

Transportation Systems Center, Environmental Measurement and Modeling Division, has 

undertaken a measurement study with the primary objective of documenting precisely the effect 

of installation location on the railroad horn sound level output at distances around the locomotive 

and inside the locomoitve cab. The sound level around the locomotive was expected to vary by 

installation location due to two factors: (1.) the sound level distribution around the locomotive 

will be affected by the presence of the locomotive body (as is the case with center-installed 

horns), and (2.) the sound level projection in all directions will be affected due to attenuation by 

the ground (as a function of height). The sound levels inside the locomotive cab due to the 

railroad horn were expected to be affected by: (1.) the distance between the horn and cab, (2.) the 

acoustic instulation of the cab, and (3.) whether the windows are open or closed. This study 

examined, through a controlled set of measurements, the sound around and inside the locomotive 

for five types of horns, installed in four locations on two models of locomotive 

This paper documents and assesses the sound level inside and around the locomotive produced 

by each horn / installation location / locomotive combination and makes observations on the 

most effective horn installations. 

2.1 HORNS, LOCOMOTIVES, AND INSTALLATION LOCATIONS 

Five horns, provided by the manufacturers, were utilized for measurements. These horns 

represent the majority of horns that are currently in use. 

(1.) Airchime K-5-LA: A five-chime horn, operating at frequencies of 311, 370, 415, 494, and 

622 Hz. The horn is rated by the manufacturer to have a sound level output of 114 dB(A) at 

100 ft with a 100 psi air supply. 

(2.) Airchime K-5-LAR24: Same as the above horn with 3 chimes facing forward and 2 chimes 

(370 and 494 Hz) facing rearward. 
3 

(3.) Leslie RS-3L: A three-chime horn, operating at frequencies of 255, 311, and 440 Hz. The 

horn is rated by the manufacturer to have a sound level output of 114 dB(A) at 100 ft with a 

100 psi air supply. 

(4.) Leslie RS-3L-RF: Same as the above horn with one chime (440 Hz) facing rearward. 

(5.) Airchime P-3: A three-chime horn, operating at frequencies of 277, 330, and 440 Hz. The 

horn is rated by the manufacturer to have a sound level output of 114 dB(A) at 100 ft with a 

100 psi air supply. 

The two locomotives used were chosen to be representative of both older (1970’s) and newer 

(1990’s) technologies. They are: (1.) an older General Motors EMD GP-40-2, Serial #786143-

1. Overall dimensions: 15.2 feet in height, 10.2 feet in width, 59.2 feet in length; and (2.) a 

newer General Motors EMD SD60MAC, BNSF #9501. Overall dimensions: 15.9 feet in height, 

10.5 feet in width, 71.6 feet in length. 

Four installation locations, described below and depicted in Figure 1, were chosen as 

representative of those either currently in service or being considered for service. Note that all 

horns were centered over the width of the locomotive. 

(1.) Center Installation 

a. GP-40. Installed on the top of the locomotive, 16 feet above ground level, 30 feet 

from the front of the locomotive. 

b. SD60MAC. Installed on the top of the locomotive, 16 feet above ground level, 40 

feet from the front of the locomotive. 

(2.) Cab Roof Installation 

a. GP-40. Installed on the top of the cab roof, 16 feet above ground level, 10 feet 

from the front of the locomotive. 

b. SD60MAC. Installed on the top of the cab roof, 16 feet above ground level, 7 

feet from the front of the locomotive. 

(3.) Front Hood Installation 

a. GP-40. Installed on the top of the front hood, 12 feet above ground level, 5 feet 

from the front of the locomotive. 

1 

2 

3 

4 

Figure 1. Horn Installation Locations 
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b. SD60MAC. Installed on the top of the front hood, 12 feet above ground level, 3 

feet from the front of the locomotive. 

(4.) Knuckle (Coupler) Installation 

a. GP-40. Installed above the coupler, 3 feet above ground level, 0 feet from the 

front of the locomotive. 

b. SD60MAC. Installed above the coupler, 3 feet above ground level, 0 feet from 

the front of the locomotive. 

To facilitate installation changes, each horn was mounted on the locomotive using a magnetic 

base, and connected to the main air reservoir (pressurized to between 130 and 140 psi) via the 

main reservior hose located on the front of the locomotive, next to the coupler. A control valve 

and pressure gauge were placed in the air line to monitor and regulate the air pressure delivered 

to the horn. 

 

2.2 SOUND MEASUREMENT LOCATIONS 

Sound level measurement instrumentation was positioned inside and around the locomotive in 

locations that would satisfy three types of measurement requirements: (1.) sound level as a 

function of location around the locomotive (directivity), at a constant distance from the horn, (2.) 

sound level per FRA certification regulations, at a constant distance from the front of the 

locomotive, and (3.) sound level inside the locomotive cab. Figure 2 is a plan view showing the 

measurement locations. 

For directivity measurements, fourteen microphones were positioned as follows: four 

microphones, located 200 and 400 ft from the front of the horn and 200 and 400 ft to the side of 

the horn (90 degrees relative to the front of the horn), were connected to Larson-Davis Model 

824 Sound Level Meter / Analyzers which were set up to measure and record both the overall Aweighted sound level and the sound level in one-third octave bands from 25 Hz to 10 kHz, at 

one-second intervals. The remaining ten microphones were positioned in a circular array at 45-

degree increments, 100 and 200 feet from the horn, and connected to Larson-Davis Model 820 

Sound Level Meters which were set up to measure and record the overall A-weighted sound 

level at one-second intervals. All distances were measured from the horn, allowing direct 

comparisons to be made between horn / locomotive / installation location configurations. 

For FRA certification measurements, two microphones were positioned at a distances of 100 and 

200 ft from the front of the locomotive, and connected to Larson-Davis Model 820 Sound Level 

Meters, which were set up to measure and record the overall A-weighted sound level at onesecond intervals. 

To measure sound levels in the locomotive cab, a microphone was connected to a Larson-Davis 

Model 820 Sound Level Meter which was set up to measure and record the overall A-weighted 

sound level at one-second intervals. The microphone was placed inside the locomotive cab, ear-
5 

level at the engineer’s typical position. Locomotive cab interior sound levels were measured 

both with the windows open (on both sides of the cab) and closed. 

Figure 2. Plan View of Measurement Locations 

2.3 TEST MATRIX 

Sound levels were measured for each horn/locomotive/installation location combination in 

accordance with three sound level/air pressure criteria. The first two criteria were such that the 

sound level output of the horn was adjusted (by adjusting the air pressure delivered to the horn) 

so that it achieved a specified level at a position 100 ft in front of the locomotive, as follows 

(constant level tests): 

(1.) the horn shall produce a level of 96 dB(A) when measured at a position 100 ft forward of the 

locomotive; and, 

(2.) the horn shall produce a level of 111 dB(A) when measured at a position 100 ft forward of 

the locomotive. 
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The third criteria was such that the horn was set to a constant air pressure (constant pressure 

tests). This allowed for accurate documentation of the barrier effect or shadow zone created by 

installing the horn in the center position on top of the locomotive. In this case, the air pressure 

delivered to the horn was 135 psi. This is the maximum pressure that could be consistently 

maintained in the main air reservoir. 

2.4 MEASUREMENT PROCEDURE 

Each acoustic measurement system was calibrated and the time-base was synchronized to a 

master clock at the beginning of each test day. Acoustic data were then measured 

simultaneously at the 17 positions for the duration of the test day. 

A total of six uncontaminated events were measured for each of the specified test criteria. An 

event was defined as a 30-second period during which the horn was sounded continuously. To 

ensure the event was acoustically uncontaminated, wind speed and direction were measured 

continuously; if the wind speed exceeded 10 mph at any time during the event, the event was 

discarded. Baseline ambient noise levels at the test site, dominated by the idling locomotive, 

were always less than 65 dB(A). Since horn sound levels exceeded 75 dB(A) even at the farthest 

measurement locations, acoustic contamination from other noise sources was not a concern. 

Because the measurement setup consisted of measurement systems that were both a constant 

distance from the horn and a constant distance from the front of the locomotive, a realignment of 

the locomotive and certification measurement systems was performed when the horn installation 

location was changed. In this manner, the horn and directivity measurement systems were kept 

at a fixed relative location for the duration of the study. 

 

3.0 ACOUSTIC DATA REDUCTION 

The contiguous, 1-second, A-weighted sound level data measured at each location were 

examined to determine the start and stop time for each event, as defined by the 10 dB-down 

period. The Maximum A-weighted Sound Level with slow time-weighting (LAsmx) and 

Equivalent A-weighted sound level (LAeq) metrics for each event were calculated and transferred 

to a spreadsheet. The six events comprising each test criteria were arithmetically averaged in the 

spreadsheet to determine a representative LAsmx and LAeq for each criteria. 

4.0 DATA ANALYSIS 

The ground surface over which the sound propagates, if it is acoustically soft (as was the case in 

this study), will generally absorb some of the sound, resulting in excess ground attenuation. The 

lower the height of the sound source, the greater the attenuation. Likewise, the locomotive body 

can effect sound propagation by diffracting the sound wave and creating a sound level reduction, 
7 

or ‘shadow’ at some locations around the locomotive; and reflecting the sound wave and creating 

sound amplification at other locations. These effects together are termed the horn installation 

effect. The sound level reduction in front of the locomotive, when the horn is installed in the 

center, is of particular concern. This reduction is greatest at close distances, decreasing with 

increasing distance. 

Because a horn operating at a fixed pressure will, theoretically, always produce the same sound 

level output at a given measurement location, the sound level data measured for the constant 

pressure test criteria data should be identical for all horn/locomotive installations. Any relative 

differences in these data are caused by the horn installation effect and/or excess ground 

attenuation. Sections 4.1 and 4.2, respectively, describe how the constant pressure sound level 

data were used to empirically derive the horn installation effect and the excess ground 

attenuation. 

4.1 HORN INSTALLATION EFFECT 

The magnitude of the sound level reduction in front of the locomotive for the center-installed 

horn was determined by comparing the sound level data measured 100, 200 and 400 ft directly in 

front of the locomotive for the center installation with the comparable data measured for the cabroof installation. Because the cab-roof installed horn is at the same height as the center-installed, 

it was assumed that there was no difference in the excess ground attenuation, and any differences 

between these installations could be attributed to the horn installation effect. A similar 

comparison was performed for the other directivity angles. 

 

Figure 3 graphically depicts the installation effect, showing the average LAeq as a function of 

angular directivity. In general, it can be seen that the presence of the locomotive body (a GP-40 

in this example) results in a 10 dB(A) reduction in sound level 100 feet from the front of the horn 

(70 feet from the front of the locomotive). For this particular horn/locomotive combination, the 

reduction in sound level drops to 9 dB(A), 200 ft from the front of the horn, and 8.1 dB(A), 400 

feet from the front of the horn. This graphic shows that there is a slight increase (0.2 to 0.7 

dB(A)) in sound level at the 45-degree position (this increase, however, is negligible) and there 

is essentially no effect at the 90-degree position. 
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Figure 3. Center-Installed vs. Cab Roof Installed Sound Levels 

Table 1 summarizes the magnitude of the installation effect for each horn / locomotive 

combination tested for the measurement positions directly in front of the locomotive. In general, 

it can be said that the magnitude of the installation effect decreases 1 to 2 dB(A) for every 

doubling of distance. Theoretically, because the line of sight to a 4-foot high receiver will 

always be blocked, the sound level reduction will never be less than about 5 dB(A)2

. 

Table 1. Summary of Installation Effect 

 Distance from Horn to 

Installation Effect dB(A) 

 Distance from Horn (ft) 

Horn Engine Front of Locomotive (ft) 100 200 400 

K-5-LA GP-40 30 -10 -9 -8.1 

K-5-LAR24 GP-40 30 -10.8 -9.1 -9.5 

RS-3L GP-40 30 -9.8 -7.9 -6.8 

K-5-LA MAC-60 40 -18.3 -15.7 -15.1 

 
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4.2 EXCESS GROUND ATTENUATION 

The reduction in sound level over distance, caused by excess ground attenuation, geometric 

spreading, and atmospheric absorption, is usually reported in terms of a drop-off rate (X dB per 

distance doubling). Geometric spreading by a point source, such as a locomotive horn, follows 

the inverse square law and results in a 20 log distance reduction in sound level, or 6 dB for each 

doubling of distance. Atmospheric absorption can be effectively neglected for the small distances 

and low frequencies examined in the current study. Any variance measured in drop-off rate from 

6 dB(A) can therefore be attributed to excess ground attenuation. 

An analysis was performed to determine the average drop-off rate for each horn installation 

location. As expected, the drop-off rate increases with decreasing horn height, that is to say that 

the drop-off rate is higher for propagation paths closer to the acoustically soft ground. The 

average drop-off rates were determined to be 5.7 dB, 6.3 dB, 7.3 dB, and 8.4 dB for the horn 

placed at heights of 16 ft (center), 16 ft (cab roof), 12 ft (front hood), and 3 ft (knuckle), 

respectively. Because the cab-roof-installed and center-installed horns were at the same height, 

the results were averaged together. As a result, for the 16 ft height, the average drop-off is 6.0 

dB; this signifies that there is no excess ground attenuation at 16 ft, whereas there is 1.3 dBand 

2.4 dB excess ground attenuation for the 12 ft and 3 ft heights, respectively. 

4.5 LOCOMOTIVE CAB INTERIOR NOISE LEVELS 

Figures 4 illustrates the average sound levels measured in the locomotive cab interior for the six 

horn / locomotive model combinations. In general, the following observations can be made: 

There is a negligible difference in interior levels between the cab roof and front hood 

positions. 

Moving the horn from the cab-roof to the center of the locomotive or the front knuckle will 

provide a substantial reduction in interior levels, usually between 3 and 11 dB(A). 

Closed windows provide between 5 and 15 dB(A) of sound level reduction. This is 

consistent with previous research on the sound level reduction of automobile windows2

. 

Levels in the newer SD60MAC average 5.5 dB(A) lower than in the older GP-40. 
10 

70

75

80

85

90

95

100

105

110

115

120

Center Cab Roof Front Hood Knuckle

Installation Location

LAeq (dB(A))

GP-40, Open, 135 psi GP-40, Closed, 135 psi

SD60MAC, Open, 135 psi SD60MAC, Closed, 135 psi

Figure 4. Locomotive Cab Interior Noise Levels 

5.0 CONCLUSIONS 

Table 2 summarizes the effect of each installation location on the sound levels measured around 

the locomoitve and inside the locomoitve cab. 

Table 2. Summary of Installation Location Effects 

Location 

Interior Sound Level 

(dB(A)), relative 

difference from cab 

roof location 

Reduction Forward 

of Locomotive 

(dB(A)) 

Excess Ground 

Attenuation (dB(A) 

per distance 

doubling) 

Center -3 to –11 -10 to –18 0 

Cab Roof 0 0 0 

Front Hood 0 0 1.3 

Knuckle -3 to –11 0 2.4 

The cab-roof location appears to best satisfy all current criterion. Due to the increased ground 

effects for both the front hood and knuckle-installations, these locations result in slightly lower 

sound levels at distance. The forward levels of the center-installed horn, due to the installation 

effect, are 8 to 10 dB lower for the GP-40 and 15 to 18 dB lower for the SD60-MAC than the 
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levels of the forward-installed (i.e., cab roof, front hood, and knuckle) horns (when operated at 

an equal pressure). 

 

In instances where a center-installation is preferred, the installation should be as far forward and 

high as possible. The measured data show a significant difference in the sound level reduction 

between the center-installed horn on the GP-40, where the horn is 30 feet from the front, and 

there is 9.5 inches between the extreme top of the locomotive and the top of the horn, and the 

SD60-MAC, where the horn is 40 feet from the front, and there is only 1 inch between the 

extreme top of the locomotive and the top of the horn (i.e., the horn is lower than some of the fan 

housings). It is recommended that the FRA continue to encourage manufacturers and the 

railroad operators to find a location for the horn that, while still removed from the cab, is as far 

front as possible, and forward of any roof-top obstructions. 

6.0 FURTHER STUDY 

Ongoing analyses will use the data measured as a part of this study to assess both the change in 

effect on community noise level and the change in effect on motorist warning. It is anticipated 

that, together, these analyses will provide the FRA with the empirical information necessary to 

write a final rule. 

7.0 REFERENCES 

1 . Federal Register, January 13, 2000. Use of Locomotive Horns at Highway-Rail Grade 

Crossings; Proposed Rule. U.S. Department of Transportation, Federal Railroad Administration. 

2. Barry, T.M., Regan, J.A., FHWA Highway Traffic Noise Prediction Model, Report No. 

FHWA-RD-77-108, Washington D.C: Federal Highway Administration, December 1978.